1. Field of the Invention
The invention is applicable in the field of pneumatic air circuits.
2. Discussion of the Related Art
Fluid control valves are common in the art. ISO 1219-1 provides the symbology for fluid control valves. Fluid control valves are often combined in fluid or air pressure circuits to control the generation of negative pressure (i.e. vacuum pressure) by porting positive fluid pressure through a Venturi vacuum generator. Vacuum pressure is used in many industrial pick-and-place applications to manually or automatically maneuver heavy or awkward pieces. If follows that control systems are designed to assist the use of vacuum pressure in such industrial applications.
FIG. 1A illustrates a prior art pressure control circuit with air-economizing capability. A Master Control Valve 8 has two states selectable either by an electrical or a mechanical actuator, an input port coupled to a Positive Pressure Source (xe2x80x9cPxe2x80x9d), and an output port. A Venturi Vacuum generator 14 has a input port, which is coupled to the output port of the Master Control Valve 8. The Venturi Vacuum generator 14 also has an exhaust port, to which a silencer is attachable, and a vacuum output port. The vacuum output port is coupled to a check valve 7, which allows only unidirectional fluid flow, and a filter 17 ultimately providing an air circuit output (xe2x80x9cVxe2x80x9d) suitable for attaching a suction head for use in industrial applications, such as pick-and-place applications. Air-economizing is provided by a pressure switch 19, which includes an electronic pressure sensor and that provides electronic feedback to the electronic actuator of the Master Control Valve 8.
During normal operating conditions, the prior art air circuit output pressure at V exhibits characteristics depicted in FIG. 1B. The Master Control Valve 8 state is toggled xe2x80x9conxe2x80x9d (t=t0) to generate vacuum pressure at V (segments xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d). Eventually, due to imperfections in the air circuit including the seal between the suction head and the work piece, the vacuum pressure decreases at xe2x80x9cVxe2x80x9d until reaching the xe2x80x9cTrip Pressurexe2x80x9d of the electronic pressure sensor 19 (segment xe2x80x9cCxe2x80x9d). When the pressure switch 19 detects that the vacuum pressure at xe2x80x9cVxe2x80x9d has reached the xe2x80x9cTrip Pressurexe2x80x9d, the pressure switch 19 output toggles the actuator of the Master Control Valve 8 and the vacuum pressure at xe2x80x9cVxe2x80x9d is restored (represented by segment xe2x80x9cDxe2x80x9d). Operation continues as illustrated in FIG. 1B until the work piece is to be release, then Blow off may be provided by additional air circuit components not illustrated in FIG. 1A.
An inherent problem of the type of control system illustrated in FIG. 1A is the dependency on electricity for software or hardware functions. More particularly, in the event of a power loss to the system, the work piece will eventually be dropped as the vacuum pressure at V diminishes beyond that required to lift the work piece and the pressure switch 19 output fails to toggle on the Master Control Valve 8. The air circuit characteristics are presented in graphical form in FIG. 1B. Thus, a problem may exist if electrical power is lost to a control system being used to maneuver a valuable item, particularly if the loss of electrical power affects a loss of generated vacuum pressure and the valuable item will be dropped.
The invention is summarized as a method and apparatus for providing a memory function for an air economizing fluid pressure circuit. A latchable or detented valve is used to control an air circuit that defaults to permit the generation of vacuum pressure, but is also cable of ceasing the generation of vacuum pressure generation to air economize. The invention may also include blow off capability for the air circuit.